This invention relates generally to gas turbine engines, and, more particularly, to structural improvements that improve the efficiency of such engines.
The superior characteristics of the gas turbine engine for numerous high power applications, both military and commercial, are well known. Low specific weight and fundamental mechanical simplicity particularly favor this type of engine.
Numerous developmental paths are available for reducing the gas turbine engine's fuel and air consumption and for reducing its size and weight. Many of these paths, however, lead to undesired complexity and high cost.
One obstacle in the evolution of the gas turbine engine is the inability to provide a sufficiently high and constant pressure ration for the compressor without unduly increasing the engine's size and cost. The graph of FIG. A shows the effects of the compressor's pressure ratio and efficiency on the engine's specific fuel and air consumption. Although an increase in the pressure ration at a given efficiency increases the engine's specific power, a more significant effect is the reduction in a specific fuel consumption.
The dramatic variation in the specific fuel consumption as a function of pressure ration is, in fact, the major disadvantage of the gas turbine engine and the Brayton cycle it follows. The engine is therefore generally unsuited for applications where power and speed are variable. It has ordinarily been used only in applications where the requirements for power and speed are substantially constant.
Another obstacle in the evolution of gas turbine engines is the limited heat resistance of the engine's fixed vanes and turbine blades. Complete combustion of petroleum fuels at a stoichiometric ratio with air results in a combustion gas temperature near 4000.degree. F. (or 2200.degree. C.). However, the engine components mentioned above cannot ordinarily be heated above about 2300.degree. F. (or 1200.degree. C.), without incurring damage. The usual solution to this overheating problem is to dilute the combustion gases with an excess of compressed air, typically there to four times as much air as is required for stoichiometric combustion with the fuel. This dilution reduces the temperature of the combustion gases below the 2300.degree. F. limit, but unfortunately requires power to compress and deliver the additional air and also significantly increases the engine's size.
An afterburner can be used to fully combust the dilution air mixed in with the combustion gases. This afterburner is located downstream of the engine's turbine, however, such that the pressure of the air delivered to it has been significantly reduced by passage through the turbine, and the afterburner's efficiency is consequently very low. The afterburner is therefore suitable for use in only very limited circumstances, for relatively short time durations, when additional power is required.
The desire to operate at higher combustion gas temperatures is longstanding. Improved metallurgy and fabrication techniques have permitted operation at somewhat higher temperatures, but there is still significant room for further improvement. The graph of FIG. B shows that increasing the combustion gas temperature at the turbine inlet merely 600.degree. F., from 1700.degree. F. to 2300.degree. F., nearly doubles the power obtained per pound of air flow.
One technique proposed for permitting engine operation at temperatures of about 2300.degree. F. involves cooling the turbine shrouding, stator blades and rotor blades using air diverted from the engine's compressor. As shown in the graph of FIG. C., this leads to inefficiencies in the thermal cycle, since a portion of the compressed air (probably about eight percent) bypasses the engine's combustion chamber.
It should therefore be appreciated that there is a significant need for a gas turbine engine that avoids the problems identified above. In particular, there is a need for an engine that combusts fuel and air at a stoichiometric ratio and that can operate at maximum efficiency for a wide range of loads. The present invention fulfills these and other needs.